In the US, it's still cheaper to sell your solar back to the utilities.

In an earnings call today, Tesla CEO Elon Musk said the company's new line of stationary storage batteries had seen “staggering” demand in less than a week. Customers have reserved more than 38,000 consumer Powerwalls and 2,500 power packs—arrays of batteries for businesses and utilities—since they became available. “Response has been overwhelming,” Musk told an audience of journalists and investors.

But Musk also admitted he thought more of Tesla's sales would ultimately come from industry and utility customers rather than to individual consumers. “We expect most of our stationary storage sales to be at the utility or industrial scale,” Musk said. The CEO added that he expects to eventually sell five- to ten-times more megawatt hours in power packs than Tesla Energy will "deploy at the consumer scale." Although Powerwalls are flying like hotcakes, individual households in the US may soon find that a Powerwall may not make much financial sense for them in the short term.

Last Thursday, Elon Musk said his company would be selling consumers 10kWh batteries at $3,500 and 7kWh batteries at $3,000. The two kinds of batteries differ in cell chemistry as well as size—the larger battery uses a nickel-cobalt mix and can only be used for backup energy storage as its chemistry won't permit frequent cycling. The smaller battery is a nickel-manganese battery that is intended for daily cycling, and if you wanted to get off the grid entirely that would be the one you'd want for evenings after the sun goes down.

Still, although a single Powerwall battery costs less than other lithium ion stationary batteries on the market, it may not be as robust as consumers want to believe. In the US, the daily cycle battery doesn't make sense for consumers—yet.

“There are two versions of the Powerwall; the daily cycling version and the power backup one,” Musk explained on the earnings call today. For people with solar panels, “the daily cycling one, it is true, in the US they are more expensive than being on the grid” and simply selling solar energy back to the utility and buying the energy back at night. “This doesn't mean that people won't buy it [in the US]. Some people want to go off-grid on principle.” Musk continued to say that the 7kWh daily cycling battery was designed with Germany and Australia in mind, where solar power is plentiful and traditional power is expensive.

Musk was in part referencing an article written by Bloomberg that said Tesla's batteries “wouldn't work well with solar.” The report claimed Tesla's main installation partner, SolarCity, was refusing to install 7kWh batteries for its customers. Jonathan Bass, Vice President of Communications for SolarCity, told Ars in a phone call this afternoon that the Bloomberg article was “completely wrong.”

Although Bass admitted SolarCity was not currently offering the daily-cycle batteries to its customers, he said that SolarCity's lack of support for the daily-cycle battery is not so bleak nor is it permanent. “If you've got solar, you're getting that metering, you get credit for that solar electricity, even if you put it back on the grid, from a financial equivalent,” Bass told Ars. The VP said that a daily-cycle battery option would be made available in 2016 for SolarCity's Hawaiian customers, where the financial trade-offs for the battery are better.

“We do expect to offer the 7kwh option in the future” to other markets, Bass told Ars.

There is room for criticism when it comes to how much power consumers may believe they're receiving. There are few use cases where a single battery could power an average household throughout a day or two of a winter snowstorm, for example. According to the US Energy Information Administration, “in 2013, the average annual electricity consumption for a US residential utility customer was 10,908 kilowatthours (kWh), an average of 909kWh per month.” Assuming you use an average amount of energy, that's still approximately 30kWh per day. In this scenario, a 10kWh back up Powerwall could get you through a few hours if the power went out at best.

The Powerwall also delivers just 2kW of power continuously. That amount is small and can max out quickly if you're trying to run a refrigerator, an electric stove, and space heaters or an air conditioning unit—as you might if power were interrupted during cold or hot weather. If you want to leave traditional power behind entirely, Bass confirmed to Ars “it would require multiple units to take someone off the grid.”

Still, Bass added 10kWh isn't bad for a backup unit. “I think it's adequate for backup for many customers. We expect it to be able to support multiple loads, refrigeration, electrical outlets, appliances, and it can be recharged by solar production.”

On the earnings call today, Musk stressed that the consumer-facing batteries were early in production and that demand for the batteries had even surprised those at Tesla. “There's no way that we could possibly satisfy this demand this year, and we're basically sold out through the middle of next year... we have to triage our responses to people who want to be a distributor.”

Tesla remains optimistic that the right markets will see an obvious use for Powerwall units. "I think we'll see demand for stationary storage to be approximate double that of the car," Musk added. "That's our best guess for long-term demand.”

Correction: This post originally stated that Tesla has had 2,800 reservations for power packs, but it has actually had 2,500.